This invention relates to a power controller circuit for a power amplifier stage, and more particularly to such a circuit which employs a linear power control circuit to supplement an exponential power control circuit.
Most applications for a power amplifier (PA) require a method for controlling the output power level. This is particularly true for any type of transmission system for wireless communications. Controlling the output power level for a power amplifier can be an especially difficult task. The accuracy of these systems is so important that they may incorporate feedback systems with additional circuitry to detect the amount of power that is actually transmitted. However, such feedback systems typically do not eliminate the need to control the output power within the PA chip, module or system block.
PA power control is usually accomplished in one of two ways: varying the input drive level or varying the gain of the PA itself. For the former, a variable gain amplifier (VGA) or voltage variable attenuator (VVA) may be added to the input of the PA to vary the input drive level. The PA then acts as a fixed gain block with its output power being roughly proportional to the input drive level. This is a technique commonly used in systems requiring linear power amplifiers, such as IS-95 (CDMA), WCDMA, CDMA2000, and IS-136 (TDMA). The VGA or VVA can be made to amplify or attenuate the signal with low distortion, and the PA can be designed to operate in the linear region since the power control does not require a change in its bias condition and mode of operation. However, a PA system that varies the input drive level can require excessive circuitry due to the use of a VGA or VVA circuit.
Alternatively and preferably, the gain of the power amplifier can be varied by changing the bias level to the power amplifier. This is a more suitable approach for systems that use saturated PAs such as GSM, or AMPS. The PA in this case receives a fixed input drive level and the transmitted output power changes proportional to the gain. The gain of the PA can be changed by adjusting the bias level of the transistors in the signal chain. This approach requires less circuitry than varying the input drive level since there is no need for a VGA or VVA, and some of the same bias control circuitry can be used, with modifications, to control the gain. It is also slightly more efficient in DC power consumption due to a change in operating class with power.
Techniques are known in the art for controlling the linear, or small signal, gain of an amplifier by varying the bias. Over the years circuits have been developed that correct for device irregularities and provide stable bias control for supply voltage and temperature variations. However, the dynamic nature of the bias for a power amplifier can eliminate the usefulness of most of these techniques.
Most PA systems are operated in a manner in which large signal behavior undesirably affects the gain-bias relationship. For example, dynamic behavior can cause DC current consumption to be higher than it would be if no signal were being amplified. Dynamic behavior also leads to a condition known as self-biasing in which the interaction between the input signal of the device and the bias controller circuitry work to raise or lower the bias point. Signal biasing occurs when the signal energy at the device base or gate creates DC current flow as the device is pushed into forward saturation or reverse breakdown, which raises or lowers the bias point of the device and changes the resulting gain. There are other phenomena such as RF leakage into the bias controller, the relationship between the bias and gain of each stage of a multi-stage amplifier, and the change in power density versus class of operation that also undesirably affect the gain-bias relationship.
The result of these undesirable effects on the gain-bias relationship is that these bias control systems are typically characterized by extremely steep and poorly defined to control curves. A typical control curve for a three stage GSM power amplifier may have a slope that is as steep as 300-500 dBV. Both the steepness of the control slope and the fact that the control slope can drop to zero as the amplifier saturates is disadvantageous for systems employing closed loop power control.
It is therefore an object of this invention to provide an improved power controller circuit for a power amplifier stage.
It is a further object of this invention to provide such an improved power controller system including a power controller circuit for one or more stages of a multistage power amplifier.
It is a further object of this invention to provide such an improved power controller circuit with reduced and extended slope of the control signal over the control range.
It is a further object of this invention to provide such an improved power controller circuit which is less complex and costly.
It is a further object of this invention to provide such an improved power controller circuit which has a marked improvement in average and peak slope of the control range.
It is a further object of this invention to provide such an improved power controller circuit which can be combined in a multistage peak amplifier to obtain an overall smoother response.
It is a further object of this invention to provide such an improved power controller circuit which can be easily implemented in a monolithic integrated circuit.
It is a further object of this invention to provide such an improved power controller which can be easily implemented using gallium arsenide HBT technology.
The invention results from the realization that an improved, less complex, and less costly power controller circuit for one or more power amplifier stages can be achieved by supplementing an exponential power control circuit with a linear power control circuit to produce a composite control current with a reduced and extended slope.
This invention features a power controller circuit for a power amplifier stage including an exponential power control circuit responsive to a power control signal for providing an exponential control current to control the power amplifier stage. A linear power control circuit responsive to the power control signal supplements the exponential control current to the power amplifier stage with a linear control current to produce a composite control current with a reduced and extended slope.
In a preferred embodiment the exponential power control circuit may include a current mirror and may include an emitter follower. The linear power control circuit may include a resistance connected to the power stage in parallel with the exponential power control circuit. The linear power control circuit may include a voltage divider circuit connected to the power stage in parallel with the exponential power control circuit. There may be a load circuit for drawing current from the power stage to reduce the slope of the composite control current at the upper end of the power control signal range. The exponential power control circuit may include a follower circuit and it may further include a second degenerative emitter follower circuit for reducing the slope of the composite control circuit at the upper end of the power control signal range.
This invention also features a power controller system for a multistage power amplifier including a power controller circuit associated with at least one of the stages and including an exponential power control circuit responsive to a power control signal for providing an exponential control current to control the power amplifier stage, and a linear power control circuit responsive to the power control signal for supplementing the exponential control current to the power amplifier stage with a linear control current to produce a composite control current with a reduced and extended slope.